Known types of contact metals used for photovoltaic (PV) mercury-cadmium-telluride (HgCdTe) IR sensors include gold (Au) with a nickel (Ni) overcoat, for individual p-type contacts, and palladium (Pd) with a Ni overcoat for n-type ground (common) contacts. However, during high temperature storage both Au and Pd are known to diffuse into HgCdTe, causing a high density of dislocations, in the case of Pd, and shorting out the p-n junction, in the case of Au. Both of these unwanted diffusions result in degraded device performance and poor high temperature (bake) stability. Furthermore, the use of Au/Ni contacts for the p-type material and Pd/Ni for the n-type material requires two separate photolithographic and deposition processes. Also, the Au/Ni and Pd/Ni metal systems each have a coefficient of thermal expansion (CTE) that differs significantly from the CTE of HgCdTe. As a result, stress is applied to the HgCdTe during thermal cycling. It is also known to employ an annealed, wide bandgap semiconductor passivation layer comprised of, by example, cadmium-telluride (CdTe) with the aforementioned contact metals. In that an as-deposited CdTe film contains residual crystal lattice stress, thermal annealing reduces the stress in the CdTe to approximately 10% of its as-deposited value. Annealing of the CdTe passivation is conventionally performed prior to depositing the contact metalization, and requires openings, or windows, to be etched through the passivation film before annealing. However, etching openings in the passivation film results in undesirable stress concentrations at the edges of the openings, thereby degrading the underlying HgCdTe material.
As a result, Dewar bake-out temperatures are typically limited to 100.degree. C. or less because of the instabilities in the surface passivation and/or the undesired diffusion of contact metal.
Furthermore, in that Hg is known to diffuse through a CdTe passivation layer during high temperature storage, a desired stoichiometric ratio of Group IIB-VIA material is not preserved within the radiation sensor device, over a prolonged period of high temperature storage.
A further problem that arises during the processing of conventional IR detector arrays relates to two unwanted chemical reactions that occur during an etch process that is used to remove oxide from indium (In) bumps, in preparation for hybridization of the array with other circuitry. By example, one suitable etch process is described in commonly assigned U.S. Pat. No. 4,865,245, issued to E.F. Schulte et al..
More specifically, it has been discovered that during the etch an electrochemical cell is formed between the In bump and the contact metal upon which the bump is deposited. The etchant, for example HCl, acts as the electrolyte. The result is that the etch rate of the In is increased in proportion to (a) the surface area of exposed contact metal, and (b) the difference in electronegativity between the contact metal and the In. As a result, the amount of In removed during the etch may vary over the surface of an array. The greatest In removal has been found to occur around the periphery of the array, where large amounts of ground contact metal are typically exposed. The degradation of the In bumps is especially troublesome for those In bumps around the outer periphery of the array, as these bumps are most prone to failure after prolonged thermal cycling of the hybrid assembly.
The second unwanted chemical reaction results in the formation of electrically conductive In-Te whiskers on the surface of the CdTe passivation layer.
The following patents are cited as relating to Group IIB-VIA photodetector fabrication techniques.
U.S. Pat. No. 4,439,912 (Pollard et al.) teaches the use of a molybdenum (Mo) layer that is overcoated with an Au/Ge layer to form connecting leads to a HgCdTe epitaxial detector array that is formed on a CdTe substrate. The leads are said to have excellent matches for the CTEs of HgCdTe and CdTe. Portions of the leads are masked prior to depositing a ZnS passivation layer. JP 60-3165(A) (Takeda) discloses, in the Abstract, the deposition, after a heat treatment, of a Mo electrode upon a (CdZnTe)(InTe) photoconductor layer. U.S. Pat. No. 4,766,084 (Bory et al.) teaches the deposition and etching of Cr and Au to form conductive pads. FIG. 2 illustrates a HgCdTe diode having an insulating layer that may be SiO.sub.2 and Si.sub.3 N.sub.4, SiO.sub.2 and ZnS, or CdTe and ZnS. In U.S. Pat. No. 4,206,003 T. Koehler describes a HgCdTe diode having a ZnS encapsulation layer that overlies an anodic oxide passivation layer. U.S. Pat. No. 4,132,999 (Maille' et al.) describes a PV detector having a HgCdTe substrate, a CdTe transition layer, and a masking layer comprised of ZnS, SiO.sub.2, SiO, or Si.sub.3 N.sub.4. After a heat treatment a window is opened to enable a doping impurity to be diffused into the HgCdTe. A protective layer of CdTe is then deposited over the masking layer and the window, Hg is diffused in through the window, an aperture is opened through the protective layer, and Cr or Au contacts are formed. U.S. Pat. No. 3,988,774 (Cohen-Solal et al.) describes a body of HgCdTe having an intermediate layer of HgCdTe or CdTe deposited thereon. Following a heat treatment windows are opened, dopants are diffused into the HgCdTe body, and Cr and Au contacts are formed. In U.S. Pat. No. 3,845,494, J. Ameurlaine et al. describe a HgCd-CdTe PV detector having Au electrodes formed on opposite faces. After forming the electrodes a Hg-impervious layer of ZnS is applied. In GB-2100927 A. Dean et al. describe a CdHgTe photodiode having a CdTe passivating layer. After a heat treatment, an Au electrode is formed through apertures in the CdTe.
What is not taught by these various U.S. and foreign Patents, and what is thus one object of this invention to provide, is a radiation sensor fabrication method, and a radiation sensor formed thereby, that deposits contact metal before the deposition of a wide bandgap semiconductor passivation layer or film, with windows to the contact metal being opened in the film after a thermal annealing step so as to significantly minimize localized stresses at the edges of the windows.
A further object of this invention is to provide a fabrication process that employs a contact metal selected to have a CTE that more closely equals that of HgCdTe, and to apply the selected contact metal prior to the deposition of a semiconductor passivation layer and a subsequent thermal anneal.
A still further object of this invention is to provide a fabrication process that significantly reduces or eliminates the possibility of unwanted chemical reactions degrading In bump interconnects during a hybridization process, thereby increasing the reliability of a resulting radiation detector hybrid assembly.